This invention pertains to the use of synthetic fibers in an interlaminar adhesive layer of a laminar wood beam, and more particularly to the use of discontinuous aramid fibers in the interlaminar adhesive layer of a laminar wood beam to improve interlaminar shear strength and to reduce creep. The invention also pertains to the use of discontinuous fibers in the adhesive or resin matrix in wood composites to enhance dimensional stability and creep resistance and the use of continuous fibers in gluelines of wood and wood composite beam structures to enhance beam creep resistance and increase strength and stiffness characteristics.
Wood and wood composite structural products (WWC) such as I-beams, glued laminated wood beams (xe2x80x9cglulamsxe2x80x9d), laminated veneer lumber (LVL), parallel strand lumber, Parallam(copyright) laminate, medium and high density fiber board, particle board, oriented strand board, flake board, and solid sawn lumber are used in a wide variety of applications from furniture to bridge girders. When placed into service, WWCs can span distances of up to 500 feet and support loads of many tons. The beams are subjected to tension, compression, and shear stress. When a laminated beam is loaded, the load causes tensile forces in some laminae and compressive forces in other laminae. For example, in a simple loading of a laminated beam with a uniform load, the lower laminae are subjected to a tensile load between support points, and the upper laminae are subjected to a compressive load between support points. This loading of the beam causes stress in the interlaminar layers of adhesive which, over time, causes creep. Creep is defined as slow, plastic deformation (inelastic or permanent deformation) under a constant load. Creep causes a vertical displacement of the beam, which displacement is referred to as sag. FIG. 1 is a load versus deflection diagram that shows creep in a reinforced glulam beam. FIG. 1 presents the results of a 24-hour creep test of a 5.125xe2x80x3xc3x9712xe2x80x3xc3x9713.5xe2x80x2 beam across a 13.5-foot span and a six-foot spread between two equal load heads.
Although a cured interlaminar layer of adhesive may be quite rigid, sustained loading of a WWC over time causes creep in the layers of adhesive between laminae or in the resin fiber composite and thereby causes the beam to sag under its load. This is an undesirable property, and beam designers try to prevent sag by such measures as over-designing the beam and adding anti-sag agents to the adhesive. Over-designing the beam is an expensive solution to the creep problem. Typically, anti-sag agents are added to the adhesive.
A typical anti-sag additive for adhesives in the wood beam industry is cellulose, which is commonly used in a granular form known as wood flour. Cellulose is added to the adhesive used between laminae to improve the shear strength of the adhesive. The shear strength of the adhesive is thus related to the interlaminar shear strength of the beam.
Any additive to an interlaminar adhesive will ideally interfere with neither the application of the adhesive nor the future use or processing of the laminate. The use of fibers as an additive to the adhesive has not been effective because the fibers interfere with the applicators used to apply the adhesive to a lamina of wood. Thus, the use of anti-sag agents in the wood industry has been confined to adhesive additives such as cellulose that do not interfere with the adhesive applicators. However, because of the increasing scarcity of wood as a resource and the related demand for higher performance wood beam structures, a more suitable solution to the problem of creep in the adhesive layer of a wood laminate is required.
Gagliani, et al. U.S. Pat. No. 4,444,823 discloses the use of an adhesive-soaked fiber mat or tow as filler and reinforcing additive in a modified polyimide adhesive. The adhesive is used to bond metal to metal, glass to glass, or ceramics to ceramics.
Schnabel U.S. Pat. No. 3,755,067 discloses the use of processed asbestos fibers to improve viscosity and thixotropic properties of a phenolic wood laminating adhesive resin.
Schijve, et al. U.S. Pat. No. 4,500,589 discloses the use of yarns of endless filaments of poly(paraphenylene terephthalamide) arranged to lie in a straight line within a resin matrix between metal sheets to create a strong composite article.
Woodbrey, et al. European Patent No. 00 013 146 discloses the fabrication of a composite article having thin aluminum layers over a relatively thick thermoplastic matrix and teaches modification of the thermoplastic matrix with various short, discontinuous fibers.
Miwa Japanese Patent No. 47(1972)-20312 discloses the use of discontinuous chopped fibers in the glueline between wood laminates to increase wood resistance to dimensional change, thereby reducing cracks in the surface of composite laminate caused by wood shrinkage stresses. The resultant composite described by Miwa does not support structural loads.
Litwin German Patent No. 3437798A1 describes the use of discontinuous fibers in the glue to achieve shear resistance in the glueline between concrete slabs. The composite does not act as a bearing component for loads across a span.
Huisman U.S. Pat. No. 2,927,623 describes a method of producing a panel in which discontinuous fibers are placed in a plastic matrix and used to form a sheet that has increased toughness and strength. The fibers are not used in gluelines as a creep or strength enhancement agent for a wood structural product.
Krys Australian Patent No. 34345178 describes a method of using discontinuous fibers in an aldehyde resin adhesive for use in manufacturing plywood to achieve higher strength and toughness. The fibers are introduced in adhesive paper to plywood and are not used as shear enhancement or as a creep reducing agent.
None of these patents discloses the use of discontinuous fibers for shear enhancement in the glueline or creep resistance in the composite structural member. Moreover, none of these patents discusses the use of continuous fibers of various lengths in the glueline for increased strength, creep resistance, and shear performance of the glueline.
One embodiment of the present invention solves the problems described above by the use of discontinuous aramid fibers in the adhesive layer of laminar wood beams. It has been found that adding aramid fibers to the adhesive layer between laminae improves the shear strength of the adhesive, reduces creep of the adhesive, and thus reduces sag of the wood beam. This embodiment of the present invention comprises the use of discontinuous aramid fibers having lengths of up to about 3 cm added to a layer of adhesive that has been applied to a laminae of wood. A second lamina of wood is then placed over the adhesive layer, and pressure is applied to adhere together the two laminae. The fiber material in the adhesive increases its gap filling properties in the interface between the two laminae.
Another embodiment of the invention is the use of discontinuous fiber material as an additive to a resin matrix for reconstituted fiber products to control dimensional change.
A third embodiment of the invention entails the use of continuous fibers in the glueline of the beam to improve its shear resistance, creep resistance, strength and stiffness characteristics.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of preferred embodiments of the invention, taken in conjunction with the accompanying drawings.